Automotive radars have been integrated into driver aids the role of which is rather to increase comfort: for example adaptive cruise controllers (ACC) for use on motorways or controllers with “Stop and Go” functionality for use in urban driving. They use microwaves and in particular the 76-81 GHz band.
Technological progress has allowed present-day applications to also target anticollision-type safety functions, and it is also envisaged in the relatively near term to achieve entirely autonomous vehicles, the environment being perceived by an association of a number of sensors based on various technologies: radar, video and infrared in particular.
Because of its all-weather nature, the radar remains in this context a sensor of key importance and its detection and discrimination capacities must be high in order to guarantee the overall reliability of the system. As regards collision prevention, the radar sensor must in particular be able to distinguish, among the stationary objects that it detects, those that correspond to elements of road infrastructure and those that correspond to vehicles parked on the road, which potentially are a collision risk. In this context, it is in particular essential that the radar does not generate false alarms liable to lead the vehicle to brake or perform an emergency avoidance manoeuvre without real cause, in particular when the vehicle is moving at high speed. This requires a high sensitivity and high discrimination capacity, allowing the situation in front of the vehicle to be sensed at large distances, typically larger than 200 m. It may also be necessary to detect the edges of roads.
In this context, distance resolution must be very high for stationary objects, this meaning that many distance boxes are needed to cover the range of the radar and therefore that the digital processing power required to carry out the processing in real time is very high. In the case of a digital beamforming radar, which must simultaneously process a plurality of angular directions, this required processing power is multiplied by the number of beams to be processed.
Moreover, as regards automotive radars, the cost of the sensor is extremely constrained, and the available computational resources are therefore limited. For an automotive application, it is therefore necessary to find ways of optimising the use of processing resources depending on the context.
One technical problem to be solved is that of obtaining a satisfactory discrimination capacity while limiting the impact on processor load. To this day, this problem has not been solved or not satisfactorily.
Automotive radars use different waveforms for short-range and long-range detection, these two modes being exclusive. These waveforms are mainly what are called frequency-modulated continuous-wave (FMCW) or frequency-shift-keying frequency-modulated continuous-wave (FSK-FMCW) waveforms. FMCW waveforms may alternate over time different frequency ramps either to optimise the emission band depending on the desired range, or to solve distance/speed ambiguity problems inherent to this type of radar. This leads to a decrease in waveform efficiency since the various emission patterns share the radar integration time.
In urban mode, at low speeds, distance resolution is given priority. It is typically less than one metre, this corresponding to a large “instantaneous” emission band, typically of several hundred megahertz. In contrast the distance domain is small, and the number of distance boxes to be processed remains modest.
On the motorway, at high speeds, speed resolution is given priority, this implying a high Doppler resolution, typically of about 25 Hz, corresponding to a discrimination of 5 cm/sec for a radar operating at 76 GHz.
The latter case corresponds to operation in ACC mode in which the distance to vehicles in front of the carrier of the radar is managed via speed. Distance resolution is relatively low, typically of a few metres, and does not allow a sufficient discrimination of obstacles to automatically engage emergency braking at high speeds. Here again, the number of distance boxes to be processed remains modest because of the low resolution, even though the distance domain is larger.
Neither of these two operating modes therefore allows a high-speed anticollision function, which requires both a high distance resolution and a high speed resolution, to be achieved.